1. Field of the Invention
Methods and apparatus for subdividing and printing regions of a substrate with multiple print heads or multiple print head assemblies which substantially decreases printing time. The methods include a feathering method which reduces overlap artifacts which is useful in any printing situation where adjacent regions are printed that could be misaligned, offset, or have slightly different colors.
2. Description of Prior Art
Inkjet printing has evolved from early systems with one nozzle which was scanned across a medium ejecting a droplet of ink when the nozzle is positioned over a to-be printed area, to systems with multiple nozzles arrayed on a Nozzleplate in a scanning Printhead (for a single color). Later multiple, different colored Printheads ganged together, and scanned as a unit called a Printhead Array. Also popular is a single printhead with a nozzle plate with up to four rows of nozzles, each of said rows ejecting one of cyan, magenta, yellow, or black.
Other implementations of inkjet printing have used Pagewidth Array heads, with the nozzle array as wide as the page or substrate to be printed. In the Pagewidth Array configuration, the Pagewidth Printhead Assembly would have at least as many nozzles as resolution elements of each color to be printed, and might consist of four Pagewidth Arrays, for each of cyan, magenta, yellow and black.
A more complete discussion of inkjet printing and print algorithms can be found in the Hewlett Packard Journal, February 1994, and available at http://www.hpl.hp.com/hpjournal/94feb/feb94.htm.
Throughout the history of inkjet printing, designers have struggled to simultaneously improve throughput, quality, and cost, which generally trade off against each other.
Many scanning head inkjet printing systems have been designed, with attention to different user needs for size of the substrate, type of substrate, speed, printing fluid, and cost. All scanning head inkjet systems have in common a basic mechanism of moving the substrate in one direction, called the paper axis in the case of conventional office printers; and moving the nozzles, printhead, or ganged printheads in a perpendicular direction, called the scan axis in the case of conventional office printers; thus enabling complete coverage of a 2 dimensional substrate with print fluid, subject to the interruption of droplet ejection by a control mechanism. The terms paper axis refers more generally to the direction that the substrate is indexed, even though the actual substrate might be fabric or other material. The term scan axis refers to the direction that printheads move. The control mechanism determines the pattern of fluid impinging on the substrate, which pattern generally can be configured to any shape, subject to various mechanical, electrical, and fluid flow limitations which can limit how accurately fluid can be placed, and how fast and accurately the scanning head or paper can be repositioned, and the rate at which fluid can be ejected.
Most scanning head ink jet printers scan a Printhead (or Printhead Array, composed of multiple printheads) laterally across a page, depositing ink in a Swath Height which is the length of the NozzlePlates Nozzle Array. Then the paper is indexed in the direction of the paper axis, and the printhead is again scanned back across the page, in the opposite direction. The printer may or may not jet ink during the retrace scan as determined by the print algorithm, which is designed to optimize and trade off print quality and speed. The index amount may be less than the full Swath Height, allowing interlacing of the ink deposited on the page to hide line feed errors and errors created by missing or mis-directed nozzles, as determined by the print algorithm. The print rate is determined by the mechanics of scanning the Printhead, and is fixed by primarily the following factors:                A. The Swath Height;        B. The scanning rate (velocity that the printhead traverses the page);        C. The number of interlaced scans necessary to achieve the desired image quality;        D. The overtravel of the printhead—which is the width of the active region of the print head or print head assembly; and        E. The acceleration and deceleration rate of the printhead.        
Inkjet printer manufacturers have struggled to get the highest print rate (pages per minute), at the lowest manufacturing cost, while still maintaining the highest quality.
To date there have been two approaches to improving print speed:                A. Adjusting parameters of the factors associated with scanning heads, described above; and        B. Using page width arrays.        
But, there is difficulty in improving scanning heads because the key parameters which affect speed are near their practical limits. For example:                A. Swath height: It is difficult to make swath height much larger than 1 inch because the paper underneath the Nozzle Array cannot be held flat enough in a region wider than that to avoid ‘head crashes’, i.e., where the printhead, as it moves, hits the paper. The inability to maintain a constant separation of the paper from the printhead can be the result of imperfections in the manufacturing tolerances of the printer, internal stresses in the paper which tend to bow it, or “cockle” which is bending due to swelling of the paper fibers as a result of absorption of the water in the ink. Swath Height can be increased by using either a printhead with a larger swath height (which are expensive to produce) or two or more printheads offset in the paper-axis direction to provide the effect of a single cartridge with wider swath height. Both implementations are subject to problems with flatness of the paper in the flat zone, among other issues, and have not found much commercial use. Examples of the staggered heads in the vertical (paper axis) direction is found in U.S. Pat. No. 5,376,958 by Brent Richtsmeier and U.S. Pat. No. 6,460,969 by Pinkernell for application to small format printers.                    FIG. 1 (prior art) shows a cut fed sheet 100, with a printable region 102. The printhead ganged set 108 is shown here without the carriage to be able to show the relative position of the various components more clearly. Printhead ganged set 108 as shown here includes printheads 106 K′, 106Y, 106K, 106C, and 106M. Each of the printheads 106 includes a nozzle array 104 that is shown as a rectangle within the outline of each printhead (such a nozzle array would only be visible from the bottom of the printhead however it is shown here to illustrate the functioning of the inkjet printer). As printhead ganged set 108 scans the page it can be seen from FIG. 1 that printhead 106K′ is positioned exactly one row higher than, and staggered to the left of, printhead 106K with staggered black cartridges 106K′ and 106K effectively simulating a single long nozzle array comprised of two nozzle arrays 104 as printhead ganged set 108 scans across the page. This can be visualized by mentally transposing printhead 106K to a position just below printhead 106K′ where it could be seen that the bottom edge of nozzle 104 of printhead 106K′ is substantially aligned with the top edge of nozzle 104 of printhead 106K.            The technique of vertically staggered printheads has been used in wide format printers such as the MacDermid Displaymaker X-12™ printer at the cost of wider paper-to-printhead separation, and hence lower print quality.                        B. Scanning rate: Scanning rate is determined by the formula: scanning rate (ips)=firing frequency/dpi (where ips is inches per second and dpi is dots per inch, the printer resolution). Scanning rates above about 30 inches per second used for 600 dpi printheads imply higher frequencies than can be supported by printhead hydraulics (e.g., maximum ink flow rate) and power limitations, especially since to increase quality, the desired number of dots per inch have also been increasing. One attempt to increase the scanning rate for black uses a second black cartridge as described by Vilanova et. al in U.S. Pat. No. 6,471,332 B1, which is incorporated in its entirety by reference here. Vilanova uses two adjacent black cartridges, firing alternately, to achieve higher scanning rates without exceeding the hydraulically limited frequency for the cartridge, and does succeed in increasing the possible scanning velocity. However, this scheme results in only minor speed improvements for office printers because the acceleration and deceleration times required to reverse direction and the increased overtravel from the addition of the second black printhead almost completely offset the increased scanning speed.        C. Also, higher scan rates require higher print head accelerations which necessitate the inclusion of large expensive motors to reverse the direction of the printhead at the ends of the scan lines in a short amount of time, results in annoying printer vibration.        D. Overtravel (i.e., the extra distance that printhead ganged set 108 must travel for all of printheads 106 in the set to move beyond the printable area of the page) cannot be reduced below 0 inches, and in any case, does not matter much once it gets significantly below the width of the printable region 102.        E. Interlacing has been dictated by the inability to cover up paper advance errors and missing or mis-directed nozzles. Little progress has been made to date in completely eliminating missing or mis-directed nozzles, although some technologies are better than others.        
On the other hand, page width arrays are expensive because they require full page widths of silicon heads for each of at least 4 colors (4×8.5 inches=34 inches of silicon currently costing at least $20/inch manufacturing cost), and the heads are sensitive to particle defects because there is no way to interlace scans as is done with scanning head printers. Thus to make an effective page width design requires redundant sets of nozzles, meaning, in reality, a viable page width array would probably require at least 3 complete sets of 4-color page width nozzle arrays. This is a very expensive proposition, and not viable for printers that are intended to sell for under a few hundred dollars.
Therefore, for black print speeds between about 18 pages per minute (achievable with 1 inch high printheads) and 100 pages per minute (achievable with page width arrays) for small format printers, there has, to date, been no cost effective inkjet solution. For purposes of this discussion, all print speeds discussed are actual print speeds for a full page of text, which is typically much slower than “specification speeds” quoted-by-manufacturers on easy-to-print, sparsely covered documents. Although, today there are inexpensive printers that claim “20 pages per minute”, this is for a sparsely printed page, typically printed in a fast, and lower quality, draft mode).
Large format printers also have reached speed limitations. They have large flat zones which limit the resolution that can be accurately printed. Further increases in print speed at high quality require an advance in the state of the art.
To achieve high quality, there are printers that interlace their scans by factors of 3 or more to hide line feed errors, or missing nozzles. This means that they advance the paper only ⅓ of the swath height, or less per scan—resulting in 3 times or more the total page printing time than would be the case if each print swath could be butted against the following swath with little overlap.
The present invention optionally incorporates a scheme for butting regions printed by different printheads without visible lines, gaps, or changes in color. Past schemes include those in two patents: U.S. Pat. No. 6,357,847 assigned to Xerox which describes using a zigzag border between the regions, and does not fully hide the borders; and U.S. Pat. No. 6,033,048 assigned to Hewlett Packard which describes a shingling/interlacing scheme for hiding line feed errors which slows printing, is inappropriate for butting of vertical regions, and incompletely hides variable overlaps.
Each of the shortcomings of the prior art noted above, as well as others, are overcome by the present invention.